Method of producing hydorgen storage alloy

ABSTRACT

A melt of a hydrogen storage alloy having an arbitrary composition is cooled gradually at a cooling rate of 5° C./min or less and solidified, alternatively an alloy having an arbitrary composition, after heating to a temperature equal to or more than a melting point thereof, is cooled gradually at a cooling rate of 5° C./min or less and solidified, and thereby a homogeneous alloy reduced in the segregation, precipitates or inclusions is obtained. An effective hydrogen storage amount, the plateau property and the durability more excellent than ever can be obtained.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to methods of producing hydrogenstorage alloys used in such as hydrogen storage materials, hydrogenabsorption materials for use in thermal conversion, hydrogen supplymaterials for use in fuel cells, negative electrode materials for use inNi-hydrogen batteries, hydrogen refining and recovering materials, andhydrogen absorption materials for use in hydrogen gas actuators, inparticular to methods of producing alloys excellent in the performanceunder environment temperatures (20 to 80° C.).

[0003] 2. Related Art

[0004] Conventionally, there has been a compressed gas system or liquidhydrogen system as an instrument for storing and transporting hydrogen,however, in place of these methods, a method that uses hydrogen storagealloys has been gathering attentions. As is generally known, thehydrogen storage alloys react reversibly with hydrogen and, inaccompanying with release and absorption of heat of reaction, absorbedand release hydrogen. By making use of the chemical reaction, a methodof storing and transporting hydrogen has been tried to put intopractical use, and furthermore, by making use of the heat of reaction,technology constituting a heat storage or heat transportation system hasbeen forwarded to develop and to put into practical use. As typicalhydrogen storage alloys, such as LaNi₅, TiFe, and TiMn_(1.5) are wellknown.

[0005] In putting various kinds of applications into practical use, theperformance of hydrogen storage materials is necessary to be furtherimproved. For instance, an increase in an amount of storable hydrogen,an improvement in the plateau property, and an improvement in thedurability can be cited as main problems.

[0006] Metals having body-centered cubic structure (hereinafter referredto as a BCC structure) such as, for instance, V, TiVMn-and TiVCr-basedalloys, have been long since known to be capable of absorbing morehydrogen than AB₅ type alloys and AB₂ type alloys that have been putinto practical use, and are considered to be promising as the hydrogenstorage alloys capable of using in the above various kinds ofapplications.

[0007] However, in the hydrogen storage alloys having the BCC structure,an amount of hydrogen that can be effectively absorbed and released issubstantially only a half that based on theory. That is, these alloysare less than sufficient in putting into practical use as the storagematerials.

[0008] That is, this kind of hydrogen storage alloys including thehydrogen storage alloys having the BCC structure are mainly producedaccording to a melting method. According to the method, thehomogenization is carried out by applying heat treatment and rapidsolidification, however, because of presence of more or lesssegregation, precipitates or inclusions, complete homogenization cannotbe attained. As a result, there is a drawback in that according to aratio of the segregation, there is observed deterioration in the plateauproperty or a decrease in the rechargeable hydrogen capacity.Furthermore, there is a practical problem in that since repetition ofthe absorption and release of hydrogen greatly damages the alloy, as thenumber of the repetition cycles increases, an equilibrium dissociationpressure largely decreases. In particular, there is tendency that in thehydrogen storage alloys having the BCC structure the drawbacks areremarkable.

SUMMARY OF THE INVENTION

[0009] According to the invention, a melt of a hydrogen storage alloy tobe finally obtained is gradually cooled at a cooling rate of 5° C./minor less and solidified, alternatively an alloy having an arbitrarycomposition, after heating to a temperature equal to or more than amelting point thereof, is gradually cooled at a cooling rate of 5°C./min or less and solidified, and thereby a homogeneous alloy that isreduced in the segregation, precipitates or inclusions can be obtained.Thereby, the invention intends to provide a hydrogen storage alloyexhibiting superior rechargeable hydrogen capacity, the plateau propertyand the durability to that of the existing ones.

[0010] In order to overcome the problems, among methods of producinghydrogen storage alloys according to the invention, a first aspect ofthe present invention is provided that a melt of a hydrogen storagealloy is solidified with gradually cooling at a cooling rate of 5°C./min or less.

[0011] According to a second aspect of the present invention, a hydrogenstorage alloy, after heating to a temperature equal to a melting pointthereof or more, is cooled gradually at a cooling rate of 5° C./min orless and thereby homogenizing treatment is applied.

[0012] According to the third aspect of the present invention, in thesecond aspect of the present invention, the hydrogen storage alloy ishomogenized by applying the application of the floating zone meltingprocess, a temperature of a floating zone melting region being in therange of from a melting point to the melting point plus 100° C., and amovement speed of the floating zone melting region being set in therange of from 1 mm/hr to 40 mm/hr.

[0013] According to a fourth aspect of the present invention, in thefirst to third aspect of the present invention, the hydrogen storagealloy has a crystal structure made of a single phase of the BCCstructure or a main phase having the BCC structure.

[0014] That is, according to the inventions, by cooling a melt graduallyat a cooling rate of 5° C./min or less to solidify, alternatively bycooling an alloy, after heating to a temperature equal to or more than amelting point thereof, gradually at a cooling rate of 5° C./min or lessto solidify, a homogeneous component composition and the suppression ofsegregation and precipitates that have not been obtained according to analloy manufacture due to the existing melting method can be obtained. Asa result, the plateau property are improved, and, furthermore, arechargeable hydrogen capacity can be increased. Still furthermore, thedeterioration of the alloy due to the repetition of absorption andrelease of hydrogen and the lowering of the equilibrium dissociationpressure can be suppressed, resulting in an improvement in thedurability of the alloy.

[0015] When the cooling rate exceeds 5° C./min, since the operationcannot be sufficiently obtained, the cooling rate is set at 5° C./min orless.

[0016] Furthermore, the cooling control and the homogenization treatmentat the production due to the melting method, when adopting one of these,can bring about the desired operations. Accordingly, the invention isnot limited by containing both constitutions.

[0017] The solidification and the homogenization of the melt, being ableto carry out according to an ordinary method except for the control ofthe cooling rate, can be carried out according to the methods of avacuum metallurgy, a floating zone melting method and a single crystalgrowth.

[0018] Furthermore, when the homogenization is applied according to thefloating zone melting in the third aspect of the present invention, bypartially melting the hydrogen storage alloy followed by re-solidifying,the operation due to the gradual cooling can be obtained. Furthermore,in the floating zone melting, inevitable impurities can be excludedoutside of the metal, and thereby the inclusions can be suppressed fromappearing. Since thus obtained alloy has a more uniform componentcomposition, all of hydrogen absorbing sites can take in hydrogen, thatis, a lot of hydrogen can be absorbed and released. Still furthermore,since a flat and large plateau can be obtained owing to the uniformcomponent composition, an amount of effectively absorbable andreleasable hydrogen can be increased. Furthermore, since metal defectsare suppressed from occurring, the deterioration due to the repetitionof the absorption and release of hydrogen can be furthermore diminished,resulting in an improvement in the durability.

[0019] In the zone melting, in order to obtain a floating zone meltingregion, a temperature is set at a melting point or more. On the otherhand, when the temperature of a molten body exceeds a melting point plus100° C., a supply balance collapses due to flowing down of a moltenmetal, the melt zone becomes difficult to maintain. Accordingly, thetemperature of the molten body is preferably set in the range of from amelting point to the melting point plus 100° C. Furthermore, accordingto the same reason as the above, the temperature of the molten body ismore preferably set at a temperature equal to or less than a meltingpoint plus 50° C. Furthermore, a movement speed of the floating zonemelting region, in order to exclude the impurities, is necessary to be 1mm/hr or more. On the other hand, when the movement speed is so largeand exceeds 40 mm/hr, the cooling rate of the alloy exceeds 5° C./min,an insufficient homogenization effect results. Accordingly, the movementspeed of the floating zone melting region is preferable to be in therange of from 1 mm/hr to 40 mm/hr. Owing to reasons similar to theabove, the movement speed is more preferable to be 20 mm/hr or less.

[0020] A hydrogen storage alloy thereto the invention can be applied isnot particularly restricted in the composition thereof, in thesolidification or the homogenization of the molten metal, a molten metalhaving an arbitrary composition or an alloy having an arbitrarycomposition can be treated as a target.

[0021] As described in the fourth aspect of the present invention, anoperation of the invention becomes remarkable particularly in thehydrogen storage alloys having a crystal structure that has a singlephase of the BCC structure or a principal phase made of the BCCstructure. As ones having a crystal structure that has a principal phasemade of the BCC structure, for instance, ones in which crystal havingthe BCC structure is 15% or more by volume can be cited. As the hydrogenstorage alloys having the BCC structure, V, TiVMn-based, TiVCr-basedalloys can be cited. However, as the invention, the hydrogen storagealloys having the BCC structure are not restricted to ones illustratedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a diagram showing a lateral sectional view of a heatingzone and a alloy in a floating zone melting furnace used in oneembodiment according to the invention.

[0023]FIG. 2 is a diagram showing an EPMA mapping image showing a stateof distribution of vanadium component in the inventive alloy.

[0024]FIG. 3 is a diagram showing an EPMA mapping image showing a stateof distribution of vanadium component in the comparative alloy.

[0025]FIG. 4 is a diagram showing a TEM image in the inventive alloy.

[0026]FIG. 5 is a diagram showing a TEM image in the comparative alloy.

[0027]FIG. 6 is a diagram showing, in comparison, equilibrium hydrogenpressure-amount of hydrogen absorption (release) isotherms in theinventive embodiment according to the invention and comparativeembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The present invention will be described hereinbelow with thereference drawings.

[0029] Raw materials are compounded so that a compositional ratiobetween Ti, V, and Cr may be 1:1:1. The compounded material is chargedin a crucible in a vacuum arc melting furnace, arc-melted under anatmosphere of high-purity argon gas, and thereafter cooled to roomtemperature in the furnace to solidify.

[0030] The melt-produced alloy is set to a floating zone melting furnaceas a alloy, the floating zone melting furnace is evacuated to 1×10⁻⁴Torr or better followed by heating the alloy in a high-purity argon gasatmosphere, and thereby the floating zone melting process is carriedout.

[0031]FIG. 1 is a schematic diagram showing the inside of the floatingzone melting furnace. In the drawing, reference numerals 1, 2, 3 and 4denote a alloy, a induction coil of the floating zone melting furnace,the floating zone melting region and a Refined alloy, respectively.

[0032] In the floating zone melting furnace, the alloy 1 is set insideof the induction coil 2, and with the induction coil 2, the floatingzone melting region 3 is heated to a temperature in the range of amelting point of the alloy 1 to the melting point plus 100° C. At theheating, the alloy 1 is moved along an axial direction of the inductioncoil 2 at a movement speed of 1 mm/hr to 40 mm/hr and the floating zonemelting region 3 is gradually cooled at a cooling rate of 5° C./min orless, and thereby the alloy 1 is refined and homogenized.

[0033] A processed hydrogen storage alloy, as needs arise, afterpulverization, can be supplied to various applications. In theembodiment, a case where the homogenization process is applied to themelt-produced hydrogen storage alloy is explained. However, by applyingthe solidification due to the gradual cooling during themelt-production, the homogenization process may be omitted,alternatively both processes may be adopted.

[0034] Preferred Embodiment

[0035] In the following, a preferred embodiment according to theinvention will be explained in comparison with a comparative example.

[0036] In the mode for carrying out the invention, the inside of thefloating zone melting furnace is evacuated to 1.0×10⁻⁴ Torr, thefloating zone melting region is heated to a temperature of a meltingpoint of the alloy plus 50° C., a movement speed of the floating zonemelting region is set at 20 mm/hr, under these conditions the floatingzone melting process is applied, and thereby an inventive alloy isobtained. Furthermore, to a alloy obtained similarly to the mode forcarrying out the invention, the homogenization process is applied at1450° C. for 60 seconds followed by water-cooling (at a cooling rate ofsubstantially 500° C./s), an obtained alloy is regarded as a comparativealloy.

[0037] In order to investigate distributions of the components in theobtained alloys, by use of the electron probe microanalysis, an electronbeam is two-dimensionally scanned, and thereby a mapping image (EPMAmapping image) of vanadium element concentration is obtained. The imagesof the inventive alloy and the comparative alloy are shown in FIGS. 2and 3, respectively.

[0038] As obvious from these drawings, in the inventive alloy, it isfound that the concentration distribution of the component is hardlyfound, that is, owing to the floating zone melting, the component ishomogenized, resulting in hardly exhibiting the segregation. On theother hand, in the comparative alloy, it is found that the distributionof the vanadium element is conspicuously observed, that is, even afterthe heat treatment, the segregation still remain.

[0039] Subsequently, in order to investigate the states of presence ofdefects such as the precipitates, inclusions and dislocation in theobtained alloys, by use of a transmission electron microscope, anelectron beam is allowed to pass through a sliced alloy, and thereby atransmitted electron image (TEM image) is obtained. FIGS. 4 and 5 showimages of the inventive alloy and the comparative alloy, respectively.

[0040] As obvious from these drawings, in the inventive alloy, it isfound that contrast that shows the presence of the defects such asprecipitates, inclusions and dislocation is hardly found, that is,components refined according to the floating zone melting process arehomogenized. On the other hand, in the comparative alloy, contrast thatshows the presence of the defects is conspicuously observed.

[0041] Furthermore, in order to investigate the hydrogen storagecharacteristics of the inventive alloy and the comparative alloy, thesealloys are pulverized to the range of substantially from 50 to 200 meshand the hydrogen absorption and release characteristics are measured ina hydrogen gas atmosphere (P (hydrogen pressure)-C (composition)-T(temperature)), and thereby a hydrogen pressure composition isothermalcurve is obtained. Still furthermore, as the comparative alloy, otherthan a sample that is water-cooled, a rapidly cooling solidified alloythat is obtained by melt-producing a material having the samecomposition as the above by use of a single roll rapidly-solidifyingdevice that is further faster in the cooling rate (1000° C./s or more inthe cooling rate) and thereby enabling to do without the homogenizingprocess is prepared. The hydrogenation properties are measured of thesesamples.

[0042] In FIG. 6, the hydrogenation properties of the inventive alloy,the comparative alloy and the rapidly cooling solidified alloy measuredat 50° C. are shown.

[0043] As obvious from FIG. 6, in the inventive alloy, owing to thehomogenization effect of the component due to the floating zone meltingprocess, an amount of absorbed hydrogen is increased by substantially20%. Furthermore, the plateau portion is also flattened, and arechargeable hydrogen capacity is increased. On the other hand, in thecomparative alloy and the rapidly cooling solidified alloy, since acontent of oxygen becomes very large, a decrease in the amount ofabsorbed hydrogen and deterioration of the plateau property areconformed.

[0044] As mentioned above, according to the invention, a hydrogenstorage alloy having an arbitrary composition, when cooled gradually ata cooling rate of 5° C./min or less, can be homogenized, accordingly, ahomogeneous alloy composition can be obtained and the segregation, theprecipitates or the inclusions can be reduced. As a result, a hydrogenstorage alloy excellent in the rechargeable hydrogen capacity, theflatness of the plateau property and the durability can be obtained.

[0045] Furthermore, when, at the homogenization, the floating zonemelting process is adopted, a temperature of a heating zone of thefloating zone melting furnace is set in the range of a melting point tothe melting point plus 100° C., and a movement speed is set in the rangeof 1 mm/hr to 40 mm/hr, a hydrogen storage alloy high in a capacity,much in an amount of absorbable and releasable hydrogen, and excellentin the durability at the repetition of the absorption and release ofhydrogen can be produced.

[0046] Still furthermore, when a hydrogen storage alloy having a crystalstructure made of a single phase of the BCC structure or mainly of theBCC structure is cooled gradually at a cooling rate of 5° C./min orless, an alloy having a more homogeneous composition than ever can beobtained. Since hydrogen can be taken in intrinsic hydrogen storagesites, a hydrogen storage alloy having a higher capacity can beproduced.

What is claimed is:
 1. A method of producing hydrogen storage alloycomprising the steps of: melting hydrogen storage alloy; cooling thehydrogen storage alloy gradually at a cooling rate of 5° C./min or less;and solidifying the hydrogen storage alloy.
 2. A method of producinghydrogen storage alloy comprising the steps of: heating a hydrogenstorage alloy to a temperature equal to a melting point thereof or more;and cooling the hydrogen storage alloy gradually at a cooling rate of 5°C./min or less with homogenizing treatment.
 3. A method of producinghydrogen storage alloy as claimed in claim 2, wherein the homogenizingtreatment includes: setting a temperature of a floating zone meltingregion in the range from a melting point to the melting point plus 100°C.; setting a movement speed of the floating zone melting region in therange of from 1 mm/hr to 40 mm/hr.
 4. A method of producing hydrogenstorage alloy as claimed in claim 1, wherein the hydrogen storage alloyhas a crystal structure made of one of a single phase of a BCC structureand a main phase having the BCC structure.
 5. A method of producinghydrogen storage alloy as claimed in claim 2, wherein the hydrogenstorage alloy has a crystal structure made of one of a single phase of aBCC structure and a main phase having the BCC structure.
 6. A method ofproducing hydrogen storage alloy as claimed in claim 3, wherein thehydrogen storage alloy has a crystal structure made of one of a singlephase of a BCC structure and a main phase having the BCC structure.